presidential candidates face off in the debate. the trump ukraine scandal has put vice president joe biden at the center of the news cycle over the last month. can he turn that to his advantage tomorrow night? or will someone else break out? jeff zeleny with a preview. reporter: good morning, dave and julia. democratic candidates heading to ohio for their fourth presidential debate. it will be on the campus in westerville, ohio. all eyes will be on joe biden and elizabeth warren and bernie the remaining u.s. troops in northern syria, pulling out, as our allies, the kurds, look to cut a deal with assad and russia. a former white house official expected to testify behind closed doors just hours from now, in the trump clearly, trying to move beyond the controversy that president trump has been stirring up, largely, without evidence. clearly, this is a defining